Prop Material - Alum boat

[sal's Dad]

I am planning to build an aluminum inboard, with a 50HP converted diesel, on a very tight budget - with used/salvaged parts wherever practical.

The prop will be about 12x12, on a 1" (or so) shaft. There seem to be plenty of parts for this setup available, used. But mostly in bronze!

I assume I need stainless steel and/or aluminum.

Any ideas for sources for this? Other thoughts or suggestions?

Thanks
Sal's Dad

[CDBarry]

Mag Brz or NiBral is very common for props on aluminum boats. People use zincs on the prop nut and sometimes the shaf. It is also common to electrically isolate the shaft (with a drivesaver, that also saves the gear, if you hit something).

[Thunderhead19]

12X12 aluminum props are pretty easy to come by, and they're dirt cheap. They aren't good for plowing sand or gravel or chopping logs though.

[sal's Dad]

I've been looking for an aluminum prop, but have only been able to find them for outboards. Am I missing something?

Can anybody suggest a source or link?

Thanks
Sal's Dad

[cyclops]

Try a propellor shop near a larger boat port. They will let you " borrow " props if you leave money.

[MikeJohns]

Originally Posted by sal's Dad

I am planning to build an aluminum inboard, with a 50HP converted diesel, on a very tight budget - with used/salvaged parts wherever practical.

The prop will be about 12x12, on a 1" (or so) shaft. There seem to be plenty of parts for this setup available, used. But mostly in bronze!

I assume I need stainless steel and/or aluminum.

Any ideas for sources for this? Other thoughts or suggestions?

Thanks
Sal's Dad

Presuming you are leaving this in seawater ?

You can use whatever you like, the issue is what is in contact with what and how are they protected.
Paint everything with a good 2 pack epoxy particularly the prop shaft and the inside of the stern tube if possible otherwise grease pack the whole tube helps, use a SS shaft a bronze or SS prop, Not aluminium), leave room for a shaft anode or if you bond then a shaft wiper connected to the hull with a decent zinc hull anode within 1m of the prop/shaft.

[globaldude]

Mike I notice you say " not aluminium " , in reference to prop choice !.
A year or so ago I was slowly driving through a local boat yard looking over the changing international cruising yachts hauled out for whatever --- as you do -- and a centre board peeking out caught my eye.
I stoped to talk to the skipper who in turn was gawping at me [ having stopped the car] to ask him if his board ever " rattled " in it's "cage" ?.

He was enthusiastic to show me why it never did . It was about 500mm wide & from memory was around 4.5 ton !!!!!, having around 2 t lead balast & a few K liters of fuel in it -------------

Sorry, I digress, anyway the thing that stuck in my head the most [ I didn't like his huge centre board that took up half the boat [ it seemed ] ] was the fact that this 20 year old yacht had a mild steel prop shaft and an aluminium prop !!!.
When I expresed amazement at same, the skipper simply said, " Misur, why not ? ".
I had no answer, especially as the proof of longevity was in front of me .
It did make me wonder, indeed - why not ?.
The shaft traveled through an oil filled " cavity" [ not sure as to it being a tube or ?? ] to which , he told me, was mounted a deep groove / thrust bearing nearest the engine and a double bearing near the prop, the out most bearing being sealed and required changing every 2nd odd year whilst hauled for anti fouling etc [ more for safety he said as they always "felt" fine]
The shaft had been blasted and painted along with the hull & was anti fouled .

So what do you think ??, with annodes ??, I woulnt have though an aloy prop would have gone 20 years under water !!.
Pete

[MikeJohns]

Pete

Would have to be well protected ( cathodically speaking ) or it will protect the more noble shaft with vigour and enthusiasm. The setup probably had an interior bonded shaft connected to an anode. Corrosion rates also come down to area ratios. Again a good paint of the whole could sufficiently protect any mismatch of metals without any other protection.

I have met oil filled sealed shaft logs before and mild steel shafts. I have seen the seals fail with some corrosion to the shaft from bronze bearing rings. I prefer grease as the filler in a short tube the power loss is not significant and it is easily applied.

Be aware that seals need a hardened bearing surface pressed over a mild steel shaft and work better on stainless shafts. The seal lip wears the shaft away in short order in softer materials ie mild s.

That keel sounds interesting.

cheers

[globaldude]

"Be aware that seals need a hardened bearing surface pressed over a mild steel shaft and work better on stainless shafts. The seal lip wears the shaft away in short order in softer materials ie mild s."
Of the sealed bearings one buys, the seal is factory fitted and "running" on the inner - ??? bit -- of the bearing. It would only remain to seal [ loctite !?] the inner "Bit" [ what's it's name ? ] of the bearing to the shaft -- and the outer to the stearn tube naturally .
I'd wondered about useing water soluble oil in the tube also - perhaps [ need to talk to oil rep ] advantages being a quick glance at the clear oil level indicator would tell you of water getting in and as it's designed to absorb water and still give lubrication up to a great dilution, would give time/ comfort to find a place to do the job .
I was led to believe the bearings were standard SKF sealed brgs ,ie steel not bronze .
Well would a steel shaft & run of the mill bronze prop fare better ?.
Pete.

[MikeJohns]

Ball bearings ? IMHO Steel ball bearing races and salt water sound highly unreliable. Better to use a sleeve bearing for reliability . Ball bearings turn into nightmares of broken bits when they fail and a prop shaft should be a bit more robust. Water soluble oils are good for metal cutting but not the best lubrication for machinery.